Questions about the grease pencil
This has interested me I must admit.

I have never really had a use for grease pencil until I saw this post and I took a little look-see in blender 2.79

It seems that if you draw a line with the grease pencil and then select it (under the grease pencil layers section in the right-hand side tab of 3D view), you can click on the convert to geometry dropdown under tools (where you select the grease pencil). The line will stay, but you can then select the line, which is now an object or curve.

And yes, you can hide the line. On the right-hand side tab in 3D view (As used above), there is a checkbox called X-Ray. Uncheck it.

Some Quick Circling to help:


EDIT: 

As of Blender 2.8, the method of using grease pencil has changed quite a bit. Firstly, to use grease pencil you must create a grease pencil object in object mode. Change to draw mode and draw your line with the tools. You can change the mode to surface, 3d cursor or origin through the drop-down middle top screen. Then switch to edit mode, and under strokes, there is the option to convert to geometry. 

Now in Blender 2.8, there is no more x-ray. It is now called depth ordering. It should be set to 3D space by default, which is like x-ray off, but you can set it to front mode for x-ray on.

Good luck with your project and I hope I helped,

BFB